Modernized packaging metadata

[agronholm]

This adds a standard pyproject.toml, allowing the package to be installed according to the instructions in README (which previously required undocumented steps like installing packaging and torch beforehand and using the --no-build-isolation flag. This flag is now documented, and build requirements have been specified. This PR also allows you to build an sdist for distribution on the PyPI by bypassing the GPU detection when building metadata only or an sdist.

[woct0rdho]

I just managed to build it in the build isolation, which is required for reproducible build and distributing wheels to others. You can see what I've done at https://github.com/woct0rdho/SageAttention

pybind11 is also required when building it. It's tricky to specify both torch (with index URL at download.pytorch.org ) and pybind11 (not in that index URL) in the build-time dependencies, and the easiest way I could think of is to use simpleindex

[agronholm]

I don't know how you did it, as when I try the same, I just get: RuntimeError: Cannot find a CUDA installation. CUDA must be available to build the package.

[agronholm]

Adding pybind11 to build dependencies does not help.

[agronholm]

Besides, don't you need to point to a torch version specific to your installed CUDA version?

[woct0rdho]

setup.py imports CUDA_HOME from torch, so you must install torch+the correct CUDA version in the build isolation

(I guess it's also possible to modify setup.py so it can build with torch+cpu downloaded from PyPI, then we don't need two index URLs. But anyway, if we want to build with nightly torch, then we still need two index URLs)

[agronholm]

setup.py imports CUDA_HOME from torch, so you must install torch+the correct CUDA version in the build isolation

Yes, that's exactly what I just said, right?

[woct0rdho]

Yes exactly

[agronholm]

Yeah, so this is why I left torch out of the build dependencies and instead opted to instruct users to install it beforehand and then disable build isolation when installing sageattention.